Replace the /dev/disk3 with the block identifier that represents the physical drive you want to access, in my case USB and FireWire drives. You can right click any drive in Disk Utility.app to find out what that is on Mac OS X or your fstab file.

This command essentially creates a pointer to the raw physical drive which you can then add to any virtual machine, just like any other virtual drive. As with creating it though, if it's mounted on the host it won't work. Also this command is undocumented and subject to change, so if the above syntax doesn't work they may have changed it since I wrote this up.

Because it enables raw file access, I'm assuming lower level tools like SpinRite can be used in a virtual machine this way, but I have to confirm this.